Large ultra-high molecular weight polyethylene spherical particles produced by AlR3 activated half-sandwich chromium(iii) catalysts

Abstract

A series of half-sandwich pentamethylcyclopentadienyl chromium(III) complexes bearing a salicylaldiminato ligand, Cp*[2-R¹-4-R²-6-(CHNR³)C₆H₂O]CrCl [R¹ = ⁱPr (1, 4), ^tBu (2, 3, 5), Ad (6); R² = H (1, 2, 3), ^tBu (4, 5, 6); R³ = ⁱPr (1, 2, 5, 6), ^tBu (3, 4)], were synthesized. All complexes were characterized by elemental analyses and the structures of complexes 1–4 and 6 were determined by X-ray diffraction analysis. These complexes adopt a pseudo-octahedral coordination environment with a three-legged piano stool geometry. Upon activation with a small amount of AlR₃, complexes 1–6 all catalyze the polymerization of ethylene in a quasi living fashion with good to high catalytic activity under mild conditions and produce ultra-high molecular weight polyethylene as spherical particles with a diameter of 1–6 mm. The catalytic activity of these complexes and the molecular weight of the produced polyethylene can be tuned in a broad range by changing the R¹, R², and R³groups as well as the AlR₃ cocatalyst. It was found that complex 6 with R¹ = Ad, R² = ^tBu, and R³ = ⁱPr shows the highest catalytic activity and produces polyethylene with the highest molecular weight.